Mad Moxxi could be a great character, but the Borderlands expansion bearing her name--Mad Moxxi's Underdome Riot--is a real dud. Taking the combat of Gearbox's first-person action-RPG and cramming it into a wave-based arena format that draws comparisons to Halo 3: ODST's Firefight mode isn't an awful idea. But by stripping away all of the character progression and loot-scavenging that gives the whole game its hook, you're left with combat for combat's sake, which makes for an extremely repetitive and empty experience.
The best thing about the entire add-on is the intro. Much like the Zombie Island expansion before it, Mad Moxxi opens with a brief, stylized sequence that introduces you to the sultry-voiced Moxxi and sets up her fetishistic desire to watch combatants in the arena. But she mostly manifests as the announcer, chiming in while you fight to let you know that a player has gone down or that another is doing well. She also makes a few movie references, which at this point is standard for Borderlands. The only real issue is that there's not enough dialogue from her, leading to a lot of repetition.
The action takes place in three different arenas, each of which look and feel like medium-sized deathmatch arenas built with assets found in other parts of the game. You and up to three other players enter the arena to take on waves of enemies, like skags, bandits, members of the Crimson Lance, guardians, and the like. Every fifth wave is a boss wave, and a curtain is literally pulled back to reveal one of the main game's named minibosses, like Nine-Toes, One-Eyed Jack, or Master McCloud. The bosses are randomized, so you never know who you'll get until they're announced.
So the game progresses from wave to wave, pausing for a bit between waves to shower you with ammo and health items, and pausing for a while longer between each round (five waves) to let you collect a few weapons. The weapons appear to be randomly generated, though in all my time in the Underdome, using characters of different levels, I never once found a gun worth using. Considering the length of time you'll spend clearing these arenas, the lack of loot is an extreme disappointment. Enemies don't drop anything at all when they die, so the guns that pop out after a boss round are all you'll get.
Your first time in each arena will be a five-round battle. This is a decent length, but by the end of these "short" tournaments, you'll probably feel like you're finished with each of the new areas. So it doesn't help much that completing this first task qualifies you for longer, 20-round tournaments. Yes, that's 100 waves of repetitive battle per location, with the difficulty slowly ratcheting up as you move from one round to the next. The content--really, just the three arenas themselves and whatever dialogue you get out of Moxxi--is stretched way too thin here.
The game attempts to throw in some variety as you proceed by adding up to four different modifiers, one of which changes after every wave. The modifiers are implemented similarly to the skulls in ODST's Firefight mode, and they make basic alterations to the action, like lower gravity, bonus damage for specific weapon types (at the expense of a damage penalty on the other weapons), headshot bonuses, no shields, or vampire, which causes your health to slowly drain when you aren't killing. Vampire, incidentally, is the subject of most of my wrath, as it happened to be active during a wave that glitched out and wouldn't end, despite all of the enemies being killed. I was left to slowly watch as everyone's health meters emptied out, killing off the entire team.
Completing Mad Moxxi's Underdome Riot will get you two additional skill points, and there's also an item bank, which lets you store any of the weird weapons you've found, but don't want to actively use. These are nice benefits, especially for players who have hit the level cap and are looking for something--anything--that will let them advance. But everything in the Underdome is a total hassle, and that gets old fast. That you don't gain any experience points or build up more weapon proficiency while in the arenas only makes a bad situation worse, and it makes the entire experience feel pretty pointless.